Find the third side in simplest radical form:

Step-by-step explanation:
[tex]by \: pythogorus \: law \\ {x}^{2} = ( { {29}^{2} - {21}^{2} })^{ \frac{1}{2} } \\ = \sqrt{20} \\ = \sqrt{4 \times 5 } \\ = 2 \sqrt{5} [/tex]